Unsteady Aerodynamic Analysis Research Articles

Numerical Simulation of Folding Tail Aeroelasticity Based on the CFD/CSD Coupling Method

This paper presents a CFD/CSD coupling method for aeroelastic simulation of folding tail morphing aircraft. The unsteady aerodynamic analysis is based on an in-house computational fluid dynamics (CFD) solver for the Euler equations, and emphasis is made on developing an efficient dynamic mesh method for the tail’s hybrid fold motion/elastic vibration deformation. The structural dynamic analysis is based on the computational structural dynamics (CSD) technique for solving the structural equation of motion in modal space. The aeroelastic coupling was achieved through successive iterations of CFD and CSD computations in the time domain. An adaptive multi-functional morphing aircraft allowing tail fold motion was selected to be studied. By using the developed method, aeroelastic simulation and mechanism analysis for fixed configurations at different folding angles and for variable configurations during the folding process were performed. The influence of folding rate on tail aeroelasticity and its influence mechanism were also analyzed.

Kinetic Equation Study on Momentum Conservation of Aerodynamics Incompressible Fluid of High-Altitude Aircraft

This paper introduces an integrated aerodynamic design method for aircraft based on numerical solution of hydrodynamics and electromagnetics equations. By using Pro/Engineer, ANSA, Fluent and MATLAB, the geometric shape, unstructured fluid mesh, hydrodynamic parameters and unpowered flight trajectory data of an aircraft were processed and calculated. In this paper, N-S equations are studied, and a test scheme for a hydrodynamic material aerodynamic model is established. Then, relevant parameters of the aircraft are tested and studied. The feasibility of this method for unsteady aerodynamics analysis of aircraft is verified.

Research on the Application of Vortex Lattice Method and Doublet Lattice Method in Aerodynamic Analysis of the Wing of Solar-Powered Unmanned Aerial Vehicle

In this paper the application of doublet lattice method and vortex lattice method is discussed. A wing of a solar-powered unmanned aerial vehicle (UAV) is chosen as the model. Both the methods are used to calculating the pressure distribution. The results show that the two methods can be used for unsteady aerodynamic analysis of such aircraft in time domain and frequency domain respectively.

Unsteady aerodynamic analysis and effectiveness of bio-inspired flapping wings in V-formation flight

Several bird species have been observed to fly in V-formation, an arrangement which exploits aerodynamic features to allow the group to conserve energy when migrating over long distances without stopping and feeding. The use of such grouping arrangement and organized pattern has demonstrated longer endurance and less power consumption in comparison with single flights. In this work, a computationally efficient potential flow solver based on the unsteady vortex lattice method (UVLM) is employed to assess the aerodynamic performance of flapping wings in forward flight in terms of lift and thrust generation along with the propulsive efficiency. The UVLM has the capability to simulate incompressible and inviscid flows over moving thin wings where the separation lines are known a priori. A bio-inspired, albatross wing shape is considered and its aerodynamic performance in formation flights is compared against conventional elliptical and rectangular wing shapes. The aerodynamic analysis is carried out for different wing arrangements of 3-body and 5-body V-formations to determine the optimal spacing parameters leading to maximum propulsive efficiency. The simulation results reveal that, at the optimal formation angle and separation distance, the albatross-inspired wing shape produces the most lift over the flapping cycle, while the rectangular wing shape generates the most thrust over the flapping cycle. Furthermore, the optimal configuration in terms of propulsive efficiency is found to be a 5-body V-formation utilizing the albatross wing shape with a separation distance set to one-third of the span and a formation angle set to 139°. The present study provides guidance for the design of multi-flapping wing air vehicles based on the expected flight mission. The albatross wing shape is found to have superior capability in producing lift, while the elliptical wing shape is observed to consume less power. The rectangular wing shape is found to produce higher thrust and then can achieve faster forward motion.

Unsteady Aerodynamic Modeling and Analysis of Load Distribution for Helicopter Rotor Blades

The complexity of the kinematics and aerodynamics of rotors is a challenging problem for achieving accurate modeling for prediction of rotor loads. The present paper describes an unsteady aerodynamic model based on strip theory for the prediction of local aerodynamic loads on helicopter blades in translational flights. The proposed model accounts aerodynamically for unsteady behavior, poststall, leading-edge suction, and, structurally, local elastic torsion. Also, the local pitching moment coefficient is calculated according to the local parameters of both aerodynamic characteristics and unsteady angle of attack. A case study with published experimental data is selected to validate the model for the same operating conditions. The case is the 7A rotor high-speed test (point 312) performed in the French Space Lab’s (ONERA) S1MA wind tunnel in Modane-Avrieux, France. The proposed model is capable of predicting, qualitatively, the variation in both local normal force and pitching moment coefficients. Computational solver results are compared with experimental results to qualitatively clarify the physical differences between them. The main conclusion is that the proposed model calculations generally reflect the same trends as the test data, providing confidence in the ability to increase the model’s fidelity in calculating the aerodynamic behavior of rotors.

Three-Dimensional Unsteady Aerodynamic Analysis of a Rigid-Framed Delta Kite Applied to Airborne Wind Energy

The validity of using a low-computational-cost model for the aerodynamic characterization of Airborne Wind Energy Systems was studied by benchmarking a three-dimensional Unsteady Panel Method (UnPaM) with experimental data from a flight test campaign of a two-line Rigid-Framed Delta kite. The latter, and a subsequent analysis of the experimental data, provided the evolution of the tether tensions, the full kinematic state of the kite (aerodynamic velocity and angular velocity vectors, among others), and its aerodynamic coefficients. The history of the kinematic state was used as input for UnPaM that provided a set of theoretical aerodynamic coefficients. Disparate conclusions were found when comparing the experimental and theoretical aerodynamic coefficients. For a wide range of angles of attack and sideslip angles, the agreement in the lift and lateral force coefficients was good and moderate, respectively, considering UnPaM is a potential flow tool. As expected, UnPaM predicts a much lower drag because it ignores viscous effects. The comparison of the aerodynamic torque coefficients is more delicate due to uncertainties on the experimental data. Besides fully non-stationary simulations, the lift coefficient was also studied with UnPaM by assuming quasi-steady and steady conditions. It was found that for a typical figure-of-eight trajectory there are no significant differences between unsteady and quasi-steady approaches allowing for fast simulations.

Effects of Bent Outlet on Characteristics of a Fluidic Oscillator with and without External Flow

A fluidic oscillator with a bent outlet nozzle was investigated to find the effects of the bending angle on the characteristics of the oscillator with and without external flow. Unsteady aerodynamic analyses were performed on the internal flow of the oscillator with two feedback channels and the interaction between oscillator jets and external flow on a NACA0015 airfoil. The analyses were performed using three-dimensional unsteady Reynolds-averaged Navier-Stokes equations with a shear stress transport turbulence model. The bending angle was tested in a range of 0–40°. The results suggest that the jet frequency increases with the bending angle for high mass flow rates, but at a bending angle of 40°, the oscillation of the jet disappears. The pressure drop through the oscillator increases with the bending angle for positive bending angles. The external flow generally suppresses the jet oscillation, and the effect of external flow on the frequency increases as the bending angle increases. The effect of external flow on the peak velocity ratio at the exit is dominant in the cases where the jet oscillation disappears.

Coupling analysis of unsteady aerodynamics and vehicle behavior with road input: Modeling and verification in road tests

Coupling analysis of unsteady aerodynamics and vehicle behavior with road input: Modeling and verification in road tests

Unsteady aerodynamics analysis and modelling of a Slingsby Firefly aircraft: Detached-Eddy Simulation model and flight test validation

This paper presents unsteady stall characteristics of a Slingsby T67M260 Firefly light aircraft from both a computational fluid dynamics (CFD) half model and flight tests. Initial results from the steady CFD, based on a RANS k−ω SST turbulence model, established the critical angle of attack of the stall to be αstall=16∘, with a maximum lift coefficient of CLmax=1.2. Comparisons with straight and level flight test data were comparable up to α=12∘ – 14∘, with the increasing deviation at higher α attributed to the effect of the propeller slipstream under these flight conditions. The RANS CFD model was then extended to an unsteady Detached-Eddy Simulation (DES) model for three angles of attack at pre-stall and stall condition (α=14∘, 16∘, 18∘), with analysis of the vortex shedding frequency. Further comparisons were then made with flight test data taken using on-board accelerometers and wing tuft surface flow visualization, at a stalled condition at equivalent α. These unsteady CFD data established a dominant shedding frequency ranging from 11.7 Hz – 8.74 Hz with increasing α and a Strouhal number based on wing chord of St = 0.11, which when compared to flight test accelerometer spectra matched within 2.9% of the measured frequency.

Unsteady aerodynamic performance analysis of an airborne wind turbine under load varying conditions at high altitude

The blade air loads of the Airborne Wind Turbine (AWT) may be significantly influenced by the unsteady flow-field at high altitude. Under these susceptible situations, the rotor needs in-depth considerations with respect to transient aspects. The present study focuses on the unsteady aerodynamic performance of stand-alone rotor under the influence of wind shear, yawed, and tilted configurations. The Computational Fluid Dynamics (CFD) transient simulations based on the sliding mesh approach are carried out for analyzing the unsteady behavior of the rotor by lifting the rotor assembly at an airborne altitude from the ground. All simulations are conducted at optimal operating conditions of wind speed and tip speed ratio. In-house Unsteady Blade Element Momentum (UBEM) code using the wind shear, dynamic stall, dynamic wake, and yaw /tilt model are applied to acquire an empirical assessment in terms of rotor torque, thrust, and power coefficient. Finally, the CFD simulated results are compared against the performance curves generated by the UBEM model and acceptable agreement is found within 4.1% deviation at peak operational conditions. The results further reveal that the time-varying aerodynamic loads on the rotor blade gradually achieve steady behavior after three rotation periods. Meanwhile, unique similarities are found in yawed and tilted inflow cases and a 10.7% loss in power coefficient is observed due to rotor yawed inflows. Additionally, in the absence of tower shadows, the strong flow interactions around the rotor blade impose a positive impact on power output. Despite the complexity of unsteady flow phenomena, this present study would be helpful to design a more proficient airborne rotor under the varying load conditions.

Nonlinear Aeroelastic Analysis of Large Wind Turbines Under Turbulent Wind Conditions

We investigate in this Paper the nonlinear aeroelastic response of large horizontal axis wind turbines under a stochastically simulated turbulent wind field, which is instantaneously sampled by the rotating and deforming blades of the rotor. The aeroelastic code combines the unsteady blade element momentum theory, equipped with several important corrections, with the displacement-based nonlinear finite element implementation of the geometrically exact beam theory. The structural model is both shearable and extensible and does not place any restrictions on the magnitudes of the configuration variables or those of the resulting one-dimensional strain measures. The aeroelastic coupling takes into account the beam axis deformations, elastic twist, and vibration-induced velocities. The three-component, three-dimensional turbulent wind field is obtained using the Mann spectrum, from which the instantaneously sampled wind speeds are extracted by spatiotemporal interpolation in the spirit of Taylor’s frozen turbulence hypothesis. The structural and unsteady aerodynamic analysis components of the aeroelastic code are validated using benchmark examples. A fully coupled nonlinear aeroelastic simulation is carried out for the National Renewable Energy Laboratory’s 5 MW research wind turbine. The influence of three-dimensional instantaneous sampling, transverse (lateral and vertical) components of turbulence, vertical wind shear, and flow unsteadiness on the aeroelastic response under turbulence is examined along with the effects of coupling parameters including the vibration-induced velocities and elastic twist of the rotor blades. The dominant frequency contributions in key aeroelastic response variables due to each of these effects are identified.

A fully coupled analysis of unsteady aerodynamics impact on vehicle dynamics during braking

This study investigates the impact of unsteady aerodynamic loads on the behavior of car dynamics during braking maneuvers. In the analysis, two independent solvers were coupled to solve an existing fluid structure interaction (FSI) problem. Transient CFD analysis applying Fluent software was used to obtain realistic aerodynamic loads. A full car, nonlinear dynamic model with elastic suspension system was built in a dedicated multibody dynamic system MSC.Adams/Car. Both physics were integrated into one ecosystem via a block diagram environment for multidomain simulation (Matlab/Simulink). Methodology elements were validated against experimental results for a rectangular beam in cross-flow with vortex-induced vibrations. The final results were validated against a full-scale experiment on a real car. The solution of an untypical flow problem (decreasing car speed during a braking maneuver) required an untypical reference frame. The classic case, with an observer at rest (stationary model and moving air), was replaced with a moving observer. The whole domain was translocated during the analysis with a velocity which varied with time. The problem was further complicated by the presence of movable aerodynamic elements. The overset mesh technique was used to allow movement of the car body and active aerodynamic surfaces. The results obtained show the importance of aerodynamic effects on the braking process. It was shown that in the analyzed scenario, movable aerodynamic features can reduce the distance to stop by 6%. The complex interaction between the active aerodynamic surface and the car body with respect to load split is discussed as well. The methodology proposed to determine the behavior car dynamics with unsteady aerodynamic effects taken into account appears to be a significant improvement when compared with existing methodologies used for such an assessment.

Effects of Structural Damage and External Stores on Transonic Flutter Stability

In this study, the structural damage and external store effects on the transonic flutter stability of a fighter type aircraft are investigated. As the damage area, the control surface-supporting structure has been selected because it is expected to affect the flutter stability sensitively. The transonic small disturbance (TSD) method is used for the analysis of unsteady aerodynamics because it is much faster than other computational fluid dynamics (CFD) schemes and is reliable for considering the control surface motions. The flutter analysis results are also compared with the results using a linear flutter analysis program to demonstrate the reliability of the analysis. This research shows that the flutter stability in the transonic regime can be further reduced after external stores are installed. This study also shows that although there is no significant decrease of flutter stability due to structural damage in subsonic regime, the flutter stability can be seriously deteriorated in the transonic regime at a certain level of damage. The effects of structural damage on the flutter stability should be carefully examined during the development of aircraft especially transonic regime.

Numerical analysis of unsteady aerodynamics of floating offshore wind turbines

The unsteady aerodynamics of a scaled model floating horizontal-axis offshore wind turbine is investigated numerically. Different aerodynamic models are used to analyze the complex unsteady flow, namely models based on unsteady Reynold-Averaged Navier-Stokes equations, free vortex wake and blade element momentum theory. The wind turbine base is subjected to imposed sinusoidal surge oscillations to reproduce the motions of the floating platform. The aerodynamic response of the rotor and the wake unsteadiness are investigated, while an evaluation of the numerical models is performed with help of wind tunnel data.

Nonlinear identification via connected neural networks for unsteady aerodynamic analysis

In the present work, a nonlinear system identification strategy is proposed which is based on the series connection of a recurrent local linear neuro-fuzzy model (NFM) and a multilayer perceptron (MLP) neural network. The NFM with output feedback is initially used for multi-step ahead predictions, whereas the MLP neural network is a posteriori employed to perform a nonlinear quasi-static correction of the NFM's time-series response. The novel identification approach is utilized exemplarily as a reduced-order modeling (ROM) technique to lower the computational effort of unsteady aerodynamic simulations, although the approach is generally applicable to any nonlinear identification task. In order to demonstrate the method's fidelity for unsteady aerodynamic modeling, the NLR 7301 airfoil is investigated at transonic flow conditions, while the motion-induced aerodynamic forces are considered in particular. Therefore, the pitch and plunge degrees of freedom are simultaneously excited via forced motions to obtain the training data for model calibration, while the respective aerodynamic response is computed using a computational fluid dynamics (CFD) solver. The sequential nonlinear identification process as well as the generalization of the resulting model is presented. Besides, a Monte-Carlo-based training procedure, which is novel in the context of aerodynamic reduced-order modeling, is introduced to estimate statistical errors. It is shown that the essential linear and nonlinear system characteristics are accurately reproduced by the new approach compared to the full-order solution. Moreover, by examining the results in comparison to established ROM methods it is indicated that the connected neural network approach leads to an enhanced simulation and generalization performance.

Unsteady loads for coaxial rotors in forward flight computed using a vortex particle method

ABSTRACTRecent advances in coaxial rotor design have shown benefits of this configuration. Nevertheless, issues related to rotor-head drag, aerodynamic performance, wake interference, and vibration should also be considered. Simulating the unsteady aerodynamic loads for a coaxial rotor, including the aerodynamic interactions between rotors and rotor blades, is an essential part of analysing their vibration characteristics. In this article, an unsteady aerodynamic analysis based on a vortex particle method is presented. In this method, a reversed-flow model for the retreating side of the coaxial rotor is proposed based on an unsteady panel technique. To account for reversed flow, shedding a vortex from the leading edge is used rather than from the trailing edge. Moreover, vortex-blade aerodynamic interactions are accounted for. The model considers the unsteady pressure term induced on a blade by tip vortices of other blades, and thus accounts for the aerodynamic interaction between the rotors and its contribution to the unsteady airloads. Coupling the reversed-flow model and the vortex-blade aerodynamic interaction model with the viscous vortex-particle method is used to simulate the complex wake of the coaxial rotor. The unsteady aerodynamic loads on the X2 coaxial rotor are simulated in forward flight, and compared with the results of PRASADUM (Parallelized Rotorcraft Analysis for Simulation And Design, developed at the University of Maryland) and CFD/CSD computations with the OVERFLOW and the CREATE-AV Helios tools. The results of the present method agree with the results of the CFD/CSD method, and compare to it better than the PRASADUM solutions. Furthermore, the influence of the aerodynamic interaction between the coaxial rotors on the unsteady airloads, frequency, wake structure, induced flow, and force distributions are analysed. Additionally, the results are also compared against computations for a single-rotor case, simulated at similar conditions as the coaxial rotor. It is shown that the effect of tip vortex interaction plays a significant role in unsteady airloads of coaxial rotors at low speeds, while the rotor blade passing effect is obviously strengthened at high-speed.

Online Dynamic Mode Decomposition Methods for the Investigation of Unsteady Aerodynamics of the DrivAer Model (First Report)

It is believed that Dynamic Mode Decomposition (DMD) is a very useful method for the analysis of unsteady aerodynamics of road vehicles. However, it seems that the conventional DMD method is not practical regarding the application on the aerodynamic design of road vehicles, since DMD computation requires massive memory. Alternatively, online DMD methods seem to be useful in practice, as those require much less memory than the conventional method. In this paper, further validation of the online DMD on the aerodynamic forces on the DrivAer model is conducted, through the comparison with results from other enhanced DMD methods and FFT.

Coupled Analysis of Unsteady Aerodynamics and Vehicle Motion of a Passenger Car in Crosswind Condition

Coupled Analysis of Unsteady Aerodynamics and Vehicle Motion of a Passenger Car in Crosswind Condition

Effect of structural dynamic characteristics on fatigue and damage tolerance of aerospace grade composite materials

In aircraft structural integrity analysis, the damage tolerance and fatigue life is investigated against a cyclic loading spectrum. The particular spectrum includes the stress/loading levels counted during a flight of certain duration. The occurrences of load factors may include higher gravitational acceleration ‘g’ levels. While maintaining a certain g level occurrence at higher angle of attack, wing structure vibrates with the amplitudes of its natural frequencies. The cyclic stress amplitudes of vibration depend upon the natural frequencies of vibrating structure, i.e. lower frequency gives higher amplitudes and vice versa.To improve the dynamic stability, modal parameters of simple carbon fibre sandwich panels have been adjusted by tailoring the fibre orientation angles and stacking sequence. In this way, the effect of change in structural dynamic characteristics on fatigue life of this simplified structure has been demonstrated. The research methodology followed in this work consists of two phases. In the first phase, aero-elastically tailored design was finalized using FEM based modal analysis and unsteady aerodynamic analysis simulations followed by experimental modal analysis. In the second phase, fatigue and damage tolerance behaviour of material was investigated using different fracture mechanics based techniques. ASTM's standard practices were adopted to determine material allowable and fracture properties. Simulation work was performed after proper calibration and correlation of finite element model with experimentally determined static and dynamic behaviour of panels.It has been observed that the applicable cyclic loading spectra, as major input parameter of fatigue analysis, largely depend upon the natural frequencies, damping and the stiffness of the structure. The results and discussions of the whole exercise may be beneficial while carrying out aero-elastic tailoring of composite aircraft wing. This research work has also a positive contribution towards multidisciplinary structural design optimization of aerospace vehicles.

The Effect of Folding Wing on Aerodynamics and Power Consumption of a Flapping Wing

Experimental study on the unsteady aerodynamics analysis and power consumption of a folding wing is accomplished using a wind tunnel testing. A folding wing model is fabricated and actuated using servo motors. The flapping wing consists of an inboard main wing and an outboard folding wing. The aerodynamic forces and consumed powers of the flapping wing are measured by changing the flapping and folding wings inside a low-speed wind tunnel. In order to calculate the aerodynamic forces, the measured forces are modified using static test data. It was found that the effect of the folding wing on the flapping wing's total lift is small but the effect of the folding wing on the total thrust is larger than the main wing. The folding motion requires the extra use of the servo motor. Thus, the amount of the energy consumption increases when both the wings are actuated together. As the flight speed increases, the power consumption of the folding wing decreases which results in energy saving.